The present invention relates to an optical attenuator of the variable attenuation type which is used to continuously adjust the attenuation of optical power transmitted through the optical attenuator, and especially to a high-performance optical attenuator of the variable attenuation type wherein unwanted optical power loss caused by reflection of optical power at the interface between a pair of optical fibers can be minimized.
Various types of optical attenuators of the variable attenuation type have been proposed and put into practical use so as to adjust the attenuation of optical power in optical fiber communication circuits.
FIG. 1 shows a side view of a conventional optical attenuator of the variable attenuation type, wherein a glass plate whose transmittance continuously changes with the angle of rotation is arranged in-between a pair of rod lenses.
A pair of rod lenses 1, 2 are arranged in an optical power transmission path 4, and a glass plate whereon a metal film is deposited so that the transmittance gradually changes with the angle of rotation is arranged in-between the pair of rod lenses 1,2.
If glass plate 3 is shifted along the optical axis or revolved around the optical axis by using an adjuster (not shown) while glass plate 3 is facing the plane perpendicular to the optical axis, the optical power along optical power transmission path 4 can be set at an arbitrary value corresponding to a specific position on glass plate 3.
The optical power reflects on the surfaces of rod lenses 1 and 2 (or at the interfaces between the air media and lens surfaces) and also at the interfaces between the air media and glass plate surfaces where a metal or metals are evaporated.
The optical power reflected from these interfaces, which is incident on the optical source, unless disregarded, affects the operation of the optical source.
The coefficient of optical power transmission for glass plate 3 in this device changes with elapsing time and this change makes the attenuation unstable.
Unlike the aforementioned method, it is also known by the person skilled in the art that the optical power can be attenuated and set at an arbitrary value which is equal to or smaller than the incident optical power by changing the distance between the tips of the optical connectors which mate together.
FIG. 2 is a cross-sectional view of an example of an optical attenuator which can continuously attenuate optical power in such a manner that the distance between the edges of ferrules which mate together is changed by an adjuster in the connector structure.
The inventor of the present invention filed U.S. Ser. No. 07/274,184, now U.S. Pat. No. 4,953,941 issued Sep. 4, 1990, for a connector structure for adjusting the distance between the edges of optical connectors as shown in FIG. 2.
Alignment adapter 5 which is finished by machine work is a cylinder made of a metal. Alignment adapter 5 has a through-hole 20 into which ferrules 14 and 15 can be inserted at the center thereof, and has an opening at one end whose inner surface provides screw 22 which mate with adjusting or setscrew nut 21.
Screws 12 and 13 which can mate with coupling nuts 8 and 9 of optical connector plugs 6 and 7 are provided on the outer surface of alignment adapter 5 at both ends thereof.
Screws 12 and 13 provide key grooves 18 and 19 which can mate with keys 16 and 17 which are arranged on the outer surfaces of ferrules 14 and 15 of optical connector plugs 6 and 7 so that ferrules 14 and 15 of optical connector plugs 6 and 7 are protected against rotating around the optical axes thereof.
Coil spring 23 is installed in a space between coupling nut 8 and ferrule 14, and coil spring 23 is installed in a space between coupling nut 9 and ferrule 15. Thus, ferrules 14 and 15 are pushed toward the edges of alignment adapter 5.
Stop rings 24 and 25 are inserted into grooves 26 and 27 of ferrules 14 and 15, respectively.
Groove 28 is provided in one side of setscrew nut 21 so as to turn setscrew nut 21 clockwise or counterclockwise.
How to adjust the attenuation of the optical attenuator of the variable attenuation type will be described briefly hereinafter.
Optical connector plugs 6 and 7, and setscrew nut 21 have been fastened to alignment adapter 5 as shown in FIG. 2. Optical fiber 29 is connected to an optical source (not shown), and optical fiber 30 is connected to an optical powermeter (not shown).
Arbitrarily set gap S between a pair of ferrule edges by adjusting the position of setscrew nut 21, set optical connector plug 7 in position, and measure the attenuation of the optical attenuator. If the attenuation is improper, repeat the above operations to alter gap S between a pair of ferrule edges until the attenuation becomes the desired value.
The operations to set the attenuation at a specified value is not easy and troublesome in this type of variable attenuator.
In addition, the optical power loss due to the reflection of optical power at the interfaces between the ferrule edges and air media is as large as 10 to 12 dB. This type of variable attenuator cannot be used for transmitting a large amount of digital information to a distant place, or for transmitting video signals to a distant place.
The objective of the present invention is to present a new type of optical attenuator of the variable attenuation type wherein the optical power loss due to the reflection of optical power at the interfaces between the ferrule edges and air media is reduced, the attenuation can easily be set at a specified value, and wherein a structure which permits easy connection of optical fibers thereto is employed.